In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. A plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided in three categories: (i) silver plates, which work according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates which contain a photopolymerizable composition that hardens upon exposure to light and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion. Thermal plates are mainly sensitized for infrared lasers emitting at 830 nm or 1064 nm. Typical photopolymer plates are sensitized for visible light, mainly for exposure by an Ar laser (488 nm) or a FD-YAG laser (532 nm). The wide-scale availability of low cost blue or violet laser diodes, originally developed for data storage by means of DVD, has enabled the production of plate-setters operating at shorter wavelength. More specifically, semiconductor lasers emitting from 350 to 450 nm have been realized using an InGaN material.
Photopolymer plates sensitized for the wavelength range from 350 to 450 nm have also been described in the prior art. Photopolymer plates generally contain a polymerizable monomer, a binder, a photoinitiator and a sensitizing dye. EP-A 985683. describes a composition comprising a titanocene compound as photoinitiator and specific dyes as sensitizers for the wavelength range from 350 to 450 nm. FP-A 1035435 discloses a 1,3 -dihydro-1-oxo-2H-indene derivative as sensitizing dye. EP-As 1048982 and 1070990 also discloses certain dyes in combination with a titanocene photoinitiator. A wide range of dyes for the wavelength range from 300 to 1200 nm is disclosed in EP-A 1091247. The sensitizing dyes disclosed in the prior art do not produce sufficient speed (sensitivity) to enable a short exposure time with the commercially available blue or violet laser diodes. Typical low-cost blue or violet laser diodes have a light output characterized by low power.
The known photopolymer printing plate precursors are unsatisfactory, because they result in printing plates of minor or even unacceptable quality because of so called pinhole defects, that have an irregular crystal-like shape. Defects called pinholes are areas having lateral dimensions of about 50 to 500 μm on the processed printing plate, that don't take up ink and therefore result in exposed areas that don't print. This unfavourable effect is particularly noticeable, if the printing plate precursor is stored before exposure and processing thereof.